International guidelines recommend supplemental oxygen if PaO2 < 6.7 kPa (50 mmHg) during air travel. Hypobaric chamber exposure is the ideal test but is not widely available. In clinical practice, the HAST has been recommended as the preferable test for pre-flight evaluation, and is used with increasing frequency. The oxygen dose during air travel has been recommended titrated during HAST. In the study, supplemental oxygen titration during hypobaric chamber exposure is compared to oxygen titration during HAST, to evaluate if HAST can be used with confidence to titrate the amount of supplemental oxygen needed during air travel.